SINGLE-IMAGE TILTED WAVE INTERFEROMETER

Description

[0001] The present invention relates to an interferometer and a method for the areal measurement of a surface or optical thickness of an optically smooth test specimen. Interferometers are suitable as precise and fast measuring devices for measuring optical surfaces. Interferometers utilize the wave properties of light. The superposition of a known reference wave and a coherent object wave, which is usually deformed by the surface of a test specimen, creates regions of extinction and regions of light amplification. The resulting intensity image, the interferogram, contains information about the deviation of the test specimen from the desired shape and can be evaluated.

[0002] An interferometer having the features of the preamble of claim 1 and a method according to the preamble of claim 10 are assumed to be known.

[0003] The known interferometer, which is described in one embodiment, for example, in [Garbusi, Eugenio; Pruss, Christof; Osten, Wolfgang: "Interferometer for precise and flexible asphere testing" in Optics letters, 33, 2973-5 (2009), 10.1364 / OL.33.002973], is used for the areal measurement of a surface or optical thickness of an optically smooth component, wherein the interferometer is configured to illuminate the optically smooth test specimen with multiple illumination configurations and to superimpose object waves influenced by the test specimen by reflection at the surface to be measured or by transmission through the test specimen with reference waves coherent with the object waves on at least one image sensor (K) to form interferograms.

[0004] Each illumination configuration results from the totality of the object waves with which the test object is simultaneously illuminated. Each object wave preferably emanates from a light-emitting point source. Therefore, an illumination configuration can also be understood as the totality of light-emitting point sources from which the aforementioned object waves emanate. An optically smooth surface is understood to be a specularly reflective surface.

[0005] The increasing use of aspherical or free-form surfaces in optical design allows for optical systems with higher image quality and more compact dimensions, but at the same time requires metrology during production. The manufacture of optics with the aforementioned accuracy requirements is only possible if the production machines receive feedback from the metrology so that the next correction step can be carried out in a targeted manner. In the area of ​​final inspection, for example, with replicated optics, surface measurements must confirm the quality of the optics and detect deviations in the manufacturing process as early as possible. Fast, flexible, yet highly accurate metrology is necessary for this feedback.

[0006] Existing solutions can be divided into scanning and area measuring systems. Scanning systems measure the test object point by point (e.g., tactile or optical coordinate measuring machines) or in individual small surface areas, which are then combined computationally (e.g., Zygo Verifire Asphere scanning interferometer, QED SSiA stitching interferometer).

[0007] Due to the inherently time-sequential approach, a stable measurement environment is required, and the measurement process extends over several minutes to several hours. This makes integration into production difficult and 100% testing impossible for high-volume production (e.g., precision injection molding of optics).

[0008] The use of so-called null optics offers great potential for rapid measurement technology. These are optics that are newly calculated and manufactured for each new type of asphere or freeform surface and adapt the wavefront of the interferometer to the specific test object. However, the potential speed advantage comes at the cost of high costs and long delivery times for the null optics.

[0009] A flexible interferometric measuring technique with which aspheres and freeform optics can be measured quickly and with low measurement uncertainty is the Tilted Wave Interferometer (TWI), invented at the Institute of Technical Optics.

[0010] The TWI achieves a measurement of the test object in less than one minute through special illumination and comprehensive detection and computational elimination of the instrument's system errors, without the need for complex zero optics.

[0011] Measuring the shape fidelity of optical components requires a resolution in the range of fractions of the wavelength A used (e.g., A / 100, with A in the visible range, for example, 400 to 800 nm), thus resulting in a resolution in the range of single-digit nanometers. This can be considered a solved problem by using area-measuring interferometers in the range of spherical or flat surfaces.

[0012] In contrast to the measurement of spherical and flat surfaces, the interferometric measurement of aspherical surfaces represents a task in optical metrology that has not yet been satisfactorily solved in many areas. The origin of the problems that arise lies in the fact that zero tests on aspheres always require the production of special refractive or diffractive optics adapted to the asphere as zero lenses.

[0013] Null lenses for aspheres are already manufactured using both diffractive and refractive methods as standard. A diffractive structure is used to reshape the spherical object wave of an interferometer lens to create a wave that is customized to the test object. However, since aspheres are manufactured in a wide variety of shapes, the production of customized null lenses is associated with significant time and expense.

[0014] None of the state-of-the-art solutions is capable of measuring an asphere or freeform surface in a single camera exposure time. This requires multiple time-sequential individual measurements, making the known methods susceptible to environmental influences. Only measurement with null optics has the potential to measure in a single camera exposure time, but this approach is too complex and expensive for many applications due to the individual null optics.

[0015] It is known that interferograms created by the superposition of two wavefronts can be evaluated using various methods. This means that the phase difference between the two wavefronts, referred to below as phase, can be determined from the intensity distributions recorded with a camera. In interferometry, this phase carries the information about the test object, which is why its determination is necessary for any interferometric measuring technique. Among the known methods for determining the phase is the very widespread phase-shift method, which requires the acquisition of several camera images. However, methods are also known in which the phase can be determined from just a single camera image.

[0016] From the abstract of WO2005052502A2, a phase difference sensor is known that measures a spatially resolved phase difference between orthogonally polarized reference and test wavefronts. The sensor is constructed as a pixelated phase mask aligned with and imaged onto a pixelated detector array. Each adjacent pixel of the phase mask measures a predetermined relative phase shift between the circularly orthogonally polarized reference and test beams. This should allow multiple phase-shifted interferograms to be synthesized simultaneously by combining pixels with identical phase shifts. The multiple phase-shifted interferograms should be able to be combined to calculate the phase difference between the reference and test wavefronts.Any configuration of an interferometer generating orthogonally polarized reference and object beams shall be capable of being combined with the phase difference sensor to provide single-shot measurements for simultaneous phase shifting.

[0017] Another method that can be used to determine the phase from only one camera image is the so-called carrier frequency method, in which the phase can be obtained according to the principle of sideband modulation, see for example [Mitsuo Takeda, Hideki Ina, and Seiji Kobayashi, "Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry," J. Opt. Soc. Am. 72, 156-160 (1982)].

[0018] The fastest flexible method to date, TWI, typically requires four phase measurements at different illumination distributions, each requiring five camera shots.

[0019] From the publication [X.Tian, ​​et al.: "Snapshot multiwavelength interference microscope, Opt. Express 26, 18279 - 18291 (2018), DOI: 10.1364 / OE.26.018279] a single-image multiwavelength interferometer for the area measurement of a surface is known, in which the surface is illuminated with light of different colors incident from the same direction and in which light emanating from the surface is decomposed into wavelength-specific partial interferograms. Tian et al. thus achieve that the information about the phase of all three wavelengths required for multiwavelength interferometry is available simultaneously in every pixel.

[0020] Against this background, the object of the invention is to provide a method and a measuring device of the type mentioned at the beginning, with which optical surfaces can be measured precisely with reduced time expenditure without zero optics.

[0021] This object is achieved in an interferometer by the features of claim 1 and in a method by the features of claim 10.

[0022] The interferometer according to the invention is characterized in that each illumination configuration is realized by a separate color channel, or by object waves with wavelengths from a contiguous wavelength range that has a wavelength-range-specific central wavelength, wherein a first of the illumination configurations has only object waves from a wavelength range that has a first central wavelength and wherein a second of the illumination configurations has only object waves from a wavelength range that has a second central wavelength, and wherein a third of the illumination configurations has only object waves from a wavelength range that has a third central wavelength, wherein the first central wavelength is different from the second central wavelength,wherein the third central wavelength is different from the first central wavelength and from the second central wavelength, and wherein the interferometer is configured to illuminate the test object simultaneously with object waves of the first illumination configuration and the second illumination configuration and the third illumination configuration from discretely different (i.e., non-parallel) directions and, after interaction with the test object, to superimpose object waves emanating from the test object on the image sensor, and wherein the image sensor is configured to spectrally decompose the interferograms resulting from the superimposition into wavelength-individual partial interferograms.

[0023] The invention parallelizes the previously used time-sequential switching of illumination configurations through the use of spectral information. Each illumination configuration is realized using its own color channel or central wavelength. This means that the structure is not illuminated by just one light source, but by, for example, three or four laser light sources of different wavelengths. The invention allows precision optics such as aspheres and freeform surfaces to be measured in the shortest possible time with low measurement uncertainty. This makes it possible to integrate the measuring process required for optics production into production, which can significantly increase productivity by saving on test piece logistics and measuring time. By capturing the entire test piece topography in a single exposure time, the influence of unstable ambient conditions is reduced to a minimum.New measurement approaches, such as the measurement of moving test pieces, are enabled. Single-image TWI measurement technology enables the interferometric measurement of variable test pieces, for example, moving test pieces in series production. Even shape-changing test pieces such as the cornea, vibrating surfaces, or the dynamic behavior of astronomical mirrors can be measured with high accuracy using the single-image TWI measurement technology according to the invention.

[0024] The ability to flexibly measure strong aspheres and freeform surfaces in a single exposure time without the use of special null optics is a new innovation. The invention succeeds in replacing the time-sequential illumination configuration changes previously required in TWI with a parallel, wavelength-separated illumination scheme, which simultaneously enables the use of single-image-based phase evaluation methods.

[0025] It is also preferred that a fourth of the illumination configurations has only object waves from a wavelength range which has a fourth central wavelength, wherein the fourth central wavelength is different from the first central wavelength, the second central wavelength and the third central wavelength, and wherein the interferometer is configured to illuminate the test object simultaneously with object waves of the first illumination configuration and the second illumination configuration and the third illumination configuration and the fourth illumination configuration from discretely different directions and, after interaction with the test object, to superimpose object waves emanating from the test object on the image recorder, and wherein the image recorder is configured to spectrally decompose the interferograms resulting from the superimposition into wavelength-individual partial interferograms.

[0026] A further preferred embodiment is characterized in that the interferometer is configured to illuminate the optically smooth component simultaneously with the first illumination configuration and the second illumination configuration, or simultaneously with the first illumination configuration and the second illumination configuration and the third illumination configuration, or simultaneously with the four illumination configurations. Each illumination configuration can comprise one object wave or a group of object waves, wherein the group can also comprise up to several hundred object waves.

[0027] Preferably, object waves propagating in adjacent directions, for example, from two point light sources arranged adjacent to each other, do not have wavelengths from the same wavelength range. In principle, configurations are also conceivable in which the point light sources are virtual point light sources.

[0028] It is further preferred that the interferometer is configured to separate interferograms by means of suitable color filters such that each pixel of the camera detector(s) essentially detects only the interferogram information of only one wavelength range.

[0029] Preferably, the object waves originate from point light sources.

[0030] A further preferred embodiment is characterized in that the point light sources are arranged such that each light source is arranged in the center of an imaginary regular hexagon, wherein the hexagons adjoin one another without gaps in a plane and that hexagons in the center of which a light source is arranged that emits light of a central wavelength or with wavelengths from one of the wavelength ranges with a central wavelength, only adjoin hexagons in the center of which a light source emitting light of a different wavelength or wavelengths from another of the wavelength ranges is arranged.

[0031] It is also preferred that the point light sources are arranged such that each light source is arranged in the center of an imaginary regular square, wherein the squares adjoin one another without gaps in a plane and wherein squares in the center of which a light source is arranged that emits light of a central wavelength or with wavelengths from one of the wavelength ranges, are only adjacent to squares in the center of which a light source emitting light of a different wavelength or light from a different wavelength range is arranged.

[0032] It is further preferred that the detector has, in addition to color filters, additional polarization filters with different orientations pixel by pixel on its individual pixels.

[0033] A further preferred embodiment is characterized in that the plurality of point light sources for each central wavelength, or for each of the wavelength ranges, or for each color channel, is an arrangement of a coherent laser source, a Kepler telescope, and a point light source array consisting of a transparent substrate, e.g., a glass substrate, on whose light entrance side facing the telescope there are microlenses whose focus lies in the plane of a light exit side of the substrate, where there is an aperture array from which object waves from one of the wavelength ranges emanate. The microlenses can, for example, be refractive, i.e., with a continuous surface, or diffractive, i.e., as Fresnel zone plates, or as a mixture of these designs.

[0034] It is also preferred that the plurality of point light sources has an arrangement of optical fiber ends for each central wavelength or for each of the wavelength ranges, which is coupled by one or more optical couplers to a laser source emitting light of the central wavelength.

[0035] Further advantages are evident from the description and the attached figures.

[0036] It is understood that the features mentioned above and those to be explained below can be used not only in the respective combinations specified, but also in other combinations or on their own. The scope of protection is defined by the claims. Drawings

[0037] Embodiments of the invention are illustrated in the drawings and explained in more detail in the following description. They show, in schematic form: Fig. 1 shows an embodiment of a known interferometer; Figure 2 shows a light exit side of a point light source array with point light sources arranged in rows and columns; Figure 3 shows three object wavefronts tilted relative to one another, incident on an optically smooth surface; Figure 4a shows outlines of an interferogram recorded with four point light sources per row and per column, with all point light sources switched on simultaneously; Figure 4b shows four patterns of interferogram patches recorded with the same interferometer, resulting from time-sequential recording; Figure 5 shows a top view of a color mosaic point light source array with a first arrangement of point light sources; Figure 6 shows a side view of a color mosaic point light source array; Figure 7 shows a side view of an alternative point light source array based on optical fibers;Figure 8 shows a plan view of a color mosaic point light source array with a second arrangement of point light sources; Figure 9 shows a Twyman-Green tilted wave interferometer; Figure 10 shows a Fizeau tilted wave interferometer; Figure 11 shows a Mach-Zehnder type interferometer; and Figure 12 shows a flowchart as an embodiment of a method according to the invention.

[0038] Figure 1shows in detail an interferometer that works with tilted wavefronts. The light from a coherent laser source L, which has only a central wavelength (defined as the wavelength of a narrow wavelength range that has the greatest intensity), is split into a test beam path and a reference beam path by a polarizing first beam splitter ST1. The light in the test beam path is expanded by a Kepler telescope consisting of a first microscope objective M1 and a collimating lens L1 and illuminates the point light source array PLQA. The PLQA consists of a quartz glass substrate with diffractive microlenses on the light entrance side facing the collimating lens. The focus of the microlenses lies in the plane of the back of the substrate, which forms a light exit side and is vapor-deposited with chromium.The chromium layer has a spatial filter aperture corresponding to each microlens, which filters out the unwanted diffraction orders of the microlenses and serves as a spatial filter for the wavefronts. Additionally, behind the PLQA is a pinhole array AA that can be moved relative to the PLQA and only transmits the light from every fourth microlens.

[0039] Figure 2 schematically shows a top view of a light exit side of the point light source array with point light sources arranged in rows and columns, each of which contains one of the microlenses and an aperture of the pinhole array. There are 1 / 4 the number of pinholes compared to microlenses.

[0040] The rows and columns are arranged at right angles to each other. The pinhole array covers every other point light source. By moving the pinhole array parallel to the rows and columns, four point light source configurations can be created. This allows for the creation of four tilted illumination configurations.

[0041] Figure 3 shows as an example three wave fronts 14.1, 14.2, 14.3 of object waves that are tilted relative to each other and are incident on an optically smooth surface.

[0042] Figure 1 further shows that light emanating from the point light sources passes through a second beam splitter ST2 and is collimated by a collimating lens L2, creating an array of wavefronts of the object waves with different tilts.

[0043] The wavefronts of the object waves are transformed into spherical wavefronts by the interferometer objective O to compensate for the fundamental curvature of the test object SUT. After reflection from the test object SUT, the wavefronts propagate back to the second beam splitter ST2, from which they are reflected into an image sensor beam path. In the Fourier plane of the interferometer 10, there is a diaphragm A with a square aperture, which filters out all areas of the wavefronts whose fringe density would exceed the Nyquist frequency of an image sensor K (e.g., a camera). After the diaphragm A, the light passes through the imaging optics AO and strikes the image sensor K.

[0044] In the reference beam path, the polarization of the light after exiting the polarizing first beam splitter ST1 is reduced by the lambda / 2 plate W by πrotated so that the polarization of the reference wave matches the polarization of the object waves (test waves) reflected by the test specimen SUT. The beam is deflected using two mirrors S1, S2, one of which is mounted on a piezo actuator PS in order to be able to evaluate the wavefronts using a phase-shifting method. The light is expanded by a Kepler telescope consisting of a second microscope objective M2 and a lens L3, and then transformed into a spherical wavefront by a lens L4. The focus of the reference wavefront is in the center of the aperture A. The light then passes through the imaging optics AO and strikes the image sensor K, in whose plane it interferes with the light of the test wavefronts.

[0045] Each point light source usually generates a measurement area on the image sensor K in the form of a small interferogram ("patch"), which can be evaluated. The pinhole array AA is shifted four times for a complete measurement. A partial measurement is performed at each of the four positions by determining the phase—that is, a phase difference between the object waves and the reference waves in the patch area caused by different optical path lengths of the object waves and the reference waves—using phase-shifting interferometry (PSI).

[0046] The spacing of the light sources 12 in the PLQA is selected such that rays from all points of the test piece surface that lie within the numerical aperture NA of the interferometer objective O arrive at the image sensor, and the areas of the image sensor covered by neighboring point light sources slightly overlap. This ensures that information about the entire test piece surface is included in one measurement.

[0047] The four partial measurements are then combined to produce the overall measurement result. The purpose of dividing the measurements into four partial measurements is to avoid overlapping of the individual areas of the different point light sources, the so-called patches of a partial measurement. This is the only way to evaluate the interferograms of the partial measurements with low measurement uncertainty.

[0048] Each illumination configuration (point light source configuration) generates zero, one, or multiple patches, depending on the shape of the test specimen. The exact distribution of the patches depends on the shape of the test specimen, its position, and the interferometer design used.

[0049] However, it is not possible to evaluate these overlapping areas using standard interferometry methods, since the resulting interferograms no longer consist of two wavefronts, but rather multi-beam interference occurs.

[0050] Fig. 4 a schematically shows an interferogram on the image sensor K, recorded with four point light sources per row and per column, with all point light sources switched on simultaneously. The interferogram patches generated with light from the individual point light sources then overlap. The interferograms cannot be evaluated in the hatched overlap areas.

[0051] Figure 4b shows four patterns of interferogram patches 18 recorded with the same interferometer, resulting when only every other point light source is switched on, for four different illumination configurations. These patterns were recorded time-sequentially, one at a time. An example of an illumination configuration results, for example, when in each row and column of a Cartesian arrangement of point light sources, only the even-numbered point light sources are switched on (other configurations: even-numbered point light sources in rows in series with odd-numbered point light sources in columns switched on; odd-numbered point light sources in rows in series with even-numbered point light sources in columns switched on; odd-numbered point light sources in rows in series with odd-numbered point light sources in columns switched on). The patches 18 then do not overlap and are therefore analyzable. The pattern of the Figure 4a results when the patches 18 of Figure 1 b are all placed on top of each other, or when all point light sources 18 with which the patches 18 of the Figure 4b generated, simultaneously contribute to the exposure of the image sensor.

[0052] The four phase measurements allow the shape deviation of the test object (SUT) from its nominal shape to be determined. Since phase-shifting interferometry, in contrast to the aforementioned carrier frequency method, which allows a phase measurement with only one image, requires at least three images for a phase measurement (typically five or more), thus requiring at least 12, or more likely 20, camera images, acquired sequentially for a complete measurement, the measurement time currently amounts to approximately half a minute. During this time, unstable environmental conditions such as vibrations, drift, and air turbulence can distort the measurement result.

[0053] In the inventive approach, instead of the time-sequential recording of interferograms each generated with a single central wavelength, a single recording of a camera image is carried out with object waves that have different wavelengths from one another and thus generate different interferograms that can be separated from one another by wavelength-dependent filtering.

[0054] In detail, the interferometer according to the invention is characterized in that a first of the object waves, which propagates in a first of the discretely different directions, has a first central wavelength and that a second of the object waves, which propagates in a second of the discretely different directions, has a second central wavelength, and that a third of the object waves, which propagates in a third of the discretely different directions, has a third central wavelength, and that the interferometer is configured to illuminate the optically smooth component simultaneously with the first object wave and the second object wave and the third object wave, or simultaneously with the four object waves. This illumination configuration results, for example, in the Figure 3 with the three object waves 14.1, 14.2, 14.3 as first, second and third object waves, whereby these object waves have different wavelengths from each other.

[0055] The basic idea of ​​the invention is to parallelize the time-sequential switching of the lighting configurations by using spectral information and thus to replace the time-shifted recording of several lighting configurations by one recording.

[0056] In the interferometer according to the invention, each illumination configuration is realized by a separate color channel, or by a wavelength range with a central wavelength. The colors, or wavelengths, are separated from the other illumination configurations / wavelengths at the image sensor K by wavelength-selective filters.

[0057] An illumination configuration is not limited to a single object wave, but can contain a multitude of discretely tilted object wavefronts. Wavefronts tilted in discretely different directions are accompanied by wave normals as propagation directions that are not parallel to each other. These object waves illuminate the test object at discretely different angles, or from discretely different and thus non-parallel directions. This means that at each point on the test object to be measured, one or more wavefronts impinge on the object, whose propagation directions differ.

[0058] This means that during exposures, the interferometer is not only illuminated by one illumination configuration, or an object wave with a central wavelength, but simultaneously by, for example, three or four object waves of different wavelengths.

[0059] Each of the laser light sources is integrated into the setup in such a way that its light illuminates the test object at different angles than the other laser light sources. This results in wavefronts as shown in the Figure 3 are shown, and which correspond to object waves with different wavelengths / colors. The desired result is the creation of colored, overlapping interferogram patches in front of or on the image sensor.

[0060] The image sensor K is preferably designed so that it can assign the resulting interferograms to their wavelength through spatially resolved wavelength selection. This allows the interfering overlaps to be resolved. An example of such an image sensor is manufactured by Sony and marketed under the name "Polysens."

[0061] For this purpose, it is necessary that the illumination angles of the individual illumination configurations are selected such that no adjacent interferogram patches 18 have the same color / wavelength, since otherwise disturbing overlapping areas with the same wavelength would arise again.

[0062] Figure 5shows a plan view of a point light source array 20 of an embodiment of an interferometer according to the invention. The point light source array 20 is characterized in that the point light sources 12 are arranged such that each point light source 12 is located at the center of an imaginary regular hexagon, wherein the hexagons are adjacent to one another without gaps in a plane, and wherein hexagons in whose center a point light source 12 emitting light with a central wavelength is arranged only border hexagons in whose center a point light source emitting light of a different wavelength is arranged. Point light sources emitting light of the same wavelength are represented by identical hatching of their associated hexagons. This makes it possible to reduce the necessary number of illumination configurations to three.

[0063] The different wavelengths can lie in different regions of a color spectrum of visible light or adjacent spectral ranges (near infrared or ultraviolet), so that the central wavelengths represent light of different colors. This applies to all embodiments and thus generally to the interferometer according to the invention.

[0064] In this arrangement, in which the point light sources are arranged on a two-dimensional grid with a hexagonal unit cell, three light sources of different wavelengths are sufficient to satisfy the condition of neighbors of different colors while fully illuminating the test specimen. When using red (e.g., 630–690 nm), green (e.g., 532 nm), and blue light (e.g., 440–460 nm), all three illumination configurations can be recorded in a single image acquisition if K RGB color cameras are used as image recorders, for example, cameras with Bayer filter arrays in front of the camera pixels. The point light source array 20 can also be referred to as a color mosaic point light source array.

[0065] Figure 6shows a side view of a color mosaic point light source array 20. The color mosaic point light source array generates point-shaped light sources, each with only one central wavelength. These are converted by the collimating lens 5 into mutually tilted, approximately planar wavefronts. The color mosaic point light source array consists of a microlens array 2, which focuses the incident light onto an aperture array 3, and an optional filter array 4, which only allows light of one of the n wavelengths to pass through each microlens. Only one of the n wavelengths is ideally focused into the aperture plane per microlens. In the preferred embodiments, n = three or four. For the Figure 5 The color mosaic point light source array shown is n = 3. Accordingly, the color mosaic point light source array generates a plurality of point light sources from n incident illumination light wavefronts of different wavelengths.

[0066] As an alternative to the microlens-based color mosaic point light source array 20 of the Figure 5 An array of optical fibers can also be used, fed by n = 3 or 4 or more light sources of different wavelengths. Figure 7 shows a point light source array based on optical fibers. Light from n = 3 lasers 6 of different wavelengths is coupled into an optical fiber 7, which is split into m fibers at a 1 / m coupler 8. The light exit ends of the fibers 6 form point light sources and are arranged as shown in the Figure 5 and 8 Here, too, point light sources emitting light of the same wavelength are represented by the same symbols. Figure 7thus shows an embodiment in which the plurality of point light sources 12 has an arrangement of optical fiber ends for each central wavelength, which is coupled by an optical coupler 8 to a laser source 6 emitting light of the central wavelength.

[0067] All point light sources 12 coupled to one of the lasers 6 form an illumination configuration. The condition that the interferogram patches do not overlap can be fulfilled by arranging them according to the Figure 5 in conjunction with the three lighting configurations, these limits must still be met. The parallel recording of these three lighting configurations in a single image is achieved by activating the three lighting configurations simultaneously, thus illuminating the test specimen simultaneously with a wavelength (or color channel) specific to each lighting configuration, and by spectrally filtering the image.

[0068] This allows the superimposed, but color-distinguishable interferogram patches 18 on the image sensor K to be separated again, for example by local color filters in front of each pixel.

[0069] As an advantageous consequence, all three / four / n illumination configurations can be registered in only one image acquisition.

[0070] Phase evaluation can traditionally be carried out via phase shifting, which, however, requires the acquisition of, for example, five individual images, which can be taken in rapid succession, so that a typical measurement time of about one second can be achieved.

[0071] Another implementation uses the previous illumination arrangement, in which the point light sources are arranged on a two-dimensional grid with a square unit cell. However, this requires four light sources of different wavelengths (or four color channels). The separation of the color wavelength channels on the image sensor can also be achieved in this case by filter arrays in front of the image sensor pixels. In this case, the filter arrays are designed so that the filters primarily transmit only one wavelength per pixel, while blocking the others.

[0072] Figure 8shows a color mosaic point light source array in which the point light sources are arranged such that each light source is located at the center of an imaginary regular square, wherein the squares are adjacent to one another without gaps in a plane, and wherein squares in the center of which a light source is arranged that emits light with a central wavelength (i.e., light from one color channel) are only adjacent to squares in the center of which a light source emitting light of a different wavelength (i.e., light from a different color channel) is arranged. Point light sources 12 that emit light of the same wavelength are represented by identical hatching of their assigned squares. This embodiment accordingly works with four lighting configurations or color channels.

[0073] Beyond the regular arrangements mentioned above, the illumination configurations can also be used with an irregular arrangement of point light sources, as long as it is ensured that no two adjacent point light sources use the same wavelength. For some test configurations, it is advantageous to position the point light sources so that they are not located on a flat surface, but rather on a curved surface, for example, or on a conical, cylindrical, or pyramidal surface.

[0074] The invention realizes the detection of the test object shape in only one camera image as follows: The light reflected or transmitted by the test object SUT (test waves or object waves reflected or transmitted by the test object SUT) is adjusted by appropriate polarizers and phase delay plates so that circularly polarized light falls on the image sensor K.

[0075] The reference wavefront required for the interference is adjusted so that counter-circularly polarized light falls on the image sensor K. By placing polarizers of different orientations in front of the pixels of the image sensor K, the relative phase between the object wave and the reference wave is varied, so that the phase to be measured can be determined using conventional phase shift algorithms.

[0076] Technically, the invention can be implemented in a very compact manner by using color sensors from Sony, which have been available for several months and which, in addition to the color filters, also have polarization filters of different orientations on the individual pixels of an image sensor.

[0077] Alternatively, instead of the polarization-based phase evaluation, carrier frequency-based methods can also be used to evaluate the phase (see [Mitsuo Takeda, Hideki Ina, and Seiji Kobayashi, "Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry," J. Opt. Soc. Am. 72, 156-160 (1982)]).

[0078] The invention is not limited to the interferometer described so far as an application example and can also be used, for example, in conjunction with a tilted wave interferometer of the Fizeau type, the Michelson type or the Mach-Zehnder type.

[0079] Figure 9 shows schematically a Twyman-Green interferometer with an illumination unit 100, a beam splitter 102, a collimation lens 104, an interferometer objective 106, a test specimen surface 108, an interferometer aperture 110, an imaging lens 112 and an image sensor K.

[0080] Figure 10 shows schematically a Fizeau tilted wave interferometer with an illumination unit 100, a beam splitter 102, a collimation lens 104, a Fizeau interferometer objective 114, a test specimen surface 108, an interferometer aperture 110, an imaging lens 112 and an image sensor K.

[0081] The interferometers described so far are operated with light reflected from the test object. Figure 11 shows a Mach-Zehnder type interferometer operated with light transmitted through the test piece with an illumination unit 100, a collimating lens 104, a first beam splitter 118, a first mirror 120, a light-transmitting test piece 122, a second beam splitter 124, an imaging lens of an interferometer aperture and an image sensor as well as with a second mirror 126 in the reference beam path.

[0082] Depending on the interferometer type, the light from an illumination unit 100 generating the required light waves reaches the transmitting test piece 122 or a reflective test piece surface 108 via various paths. The illumination unit 100 is, for example, a color mosaic point light source array 20 as described above. There, the light interacts with the test piece 122 / the test piece surface 108 and thereby carries the desired information. For example, a Twyman-Green or Fizeau-type setup can be selected if the surface topography of the test piece surface is to be measured, or a Mach-Zehnder-type setup is selected and the test piece used in transmission if the quantity to be measured is its optical thickness.After interaction with the test object, the resulting test wavefronts of different wavelengths reach the image sensor K, where they are superimposed with the respective coherent reference wavefronts. By using light of different wavelengths, all point light sources can emit light simultaneously during the image sensor's exposure time, for at least a fraction of the exposure time. Since all point light sources are switched on simultaneously, the measurement information for the entire test object is captured simultaneously in one image sensor image.

[0083] The spectral separation of the interferograms resulting from the superposition on the image sensor K can be achieved using various technical implementations. In addition to the filter masks in front of the pixels of an image sensor mentioned above, the spectral channels can be realized using separate cameras / image sensors for each color or central wavelength. In this case, the light from the test object is distributed to the various image sensors by beam splitters and is either spectrally selected at the beam splitters using dichroic beam splitter layers or spectrally selected directly in front of the image sensors using absorbing filters.

[0084] The interferogram patches separated by the color filters initially show sinusoidal intensity stripes, from which the phase difference and thus the path difference between the reference and object wave, from which the test object properties to be measured, are calculated during the phase evaluation.

[0085] According to the state of the art, phase evaluation can be performed using temporal phase shifting, i.e., by recording a sequence of interferograms in which a phase change between the object and reference wave is deliberately introduced between the recordings, typically by moving a mirror in the beam path. This requires the acquisition of, for example, five individual images, which can be acquired in rapid succession, allowing a typical measurement time of approximately one second.

[0086] The invention achieves the following: The spectrally superimposed interferograms of the three or more illumination configurations are split into individual interferograms, preferably using spectral selection, with each individual interferogram corresponding to an illumination configuration. These individual interferograms are each evaluated individually using individual interferogram evaluation methods. A preferred embodiment of individual interferogram evaluation is evaluation using polarization methods (e.g., US 7,777,895, US 6,304,330, US 6,552,808, US 7,230,717). The light from the test specimen is adjusted by polarizers and phase delay plates so that circularly polarized light strikes the image sensor. The reference wavefront required for the interference is adjusted so that counter-propagating circularly polarized light strikes the image sensor.By placing polarizers of different orientation in front of the pixels of the image sensor, the relative phase between the object wave and the reference wave is varied, so that the phase to be measured can be determined from a group of four pixels using conventional phase shift algorithms.

[0087] Technically, the invention can be realized very compactly by using the polarization image sensors "Polarsens" from Sony, which have been available for several months and which, in addition to the color filters, also have polarization filters with four different orientations on the individual pixels, whereby the polarization filters are arranged between a photodiode layer and a lens layer of the image sensor chip.

[0088] In a further preferred embodiment, the phase of the individual interferograms is determined using carrier frequency methods, see, for example, [Mitsuo Takeda, Hideki Ina, and Seiji Kobayashi, "Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry," J. Opt. Soc. Am. 72, 156-160 (1982)]. Here, the object waves are superimposed with reference waves that are strongly tilted relative to the object waves. The phase evaluation is then performed using demodulation methods from the high-frequency interferogram fringes.

[0089] Figure 12 shows a flow chart as an embodiment of a method according to the invention for the areal measurement of a surface or optical thickness of an optically smooth test specimen.

[0090] In a first step 130, the optically smooth test specimen is illuminated with a plurality of object waves from discretely different directions. A first of the object waves, incident from a first direction, has a first central wavelength. A second of the object waves, propagating in a second of the discretely different directions, has a second central wavelength, and a third of the object waves, propagating in a third of the discretely different directions, has a third central wavelength. The first central wavelength is different from the second central wavelength, and the third central wavelength is different from the first central wavelength and the second central wavelength. The test specimen is illuminated simultaneously with the first object wave and the second object wave and the third object wave.After interaction with the test object, object waves emanating from it are superimposed on the image sensor in a step 132. In a third step 134, the interferograms resulting from the superposition are spectrally decomposed into wavelength-specific sub-interferograms.

[0091] In a preferred embodiment, the interferometer 10 is configured to illuminate the optically smooth test specimen with multiple illumination configurations, each illumination configuration resulting from a plurality of object waves with which the test specimen is illuminated, and to superimpose object waves influenced by the test specimen by reflection at the surface 108 to be measured or by transmission through the test specimen 122 with reference waves coherent with the object waves on at least one image sensor K to form interferograms. Each illumination configuration is realized by its own color channel or its own central wavelength, wherein a first of the illumination configurations has only object waves 14.1, 14.2, 14.3 that have a first central wavelength, and wherein a second of the illumination configurations 14.1, 14.2, 14.3 only has object waves having a second central wavelength, wherein a third of the illumination configurations only has object waves 14.1, 14.2, 14.3, which have a third central wavelength, wherein the first central wavelength is different from the second central wavelength, wherein the third central wavelength is different from the first central wavelength and from the second central wavelength, and wherein the interferometer 10 is configured to illuminate the test object 122 simultaneously with object waves of the first illumination configuration and the second illumination configuration and the third illumination configuration from discretely different directions and, after interaction with the test object 122, to superimpose object waves emanating from the latter on the image recorder K, and wherein the image recorder K is configured to spectrally decompose the interferograms resulting from the superimposition into wavelength-individual partial interferograms.

[0092] It is also preferred that a fourth of the illumination configurations comprises only object waves having a fourth central wavelength and that the fourth central wavelength is different from the first central wavelength and from the second central wavelength and from the third central wavelength.

[0093] It is further preferred that object waves propagating in directions closest to each other do not have the same central wavelength.

[0094] A further preferred embodiment is characterized in that the image sensor K is configured to separate interferograms by color filters such that each pixel of the image sensor K detects only the interferogram information of only one wavelength.

[0095] It is also preferred that the interferometer has a plurality of point light sources 12 from which the object waves of an illumination configuration emanate simultaneously.

[0096] It is further preferred that the point light sources 12 are arranged such that each point light source 12 is arranged in the center of an imaginary regular hexagon, wherein the hexagons adjoin one another without gaps in a plane and that hexagons in the center of which a point light source 12 is arranged, which emits light with a central wavelength, only adjoin hexagons in the center of which a point light source 12 emitting light of a different wavelength is arranged.

[0097] A further preferred embodiment is characterized in that the point light sources 12 are arranged such that each point light source 12 is arranged in the center of an imaginary regular square, wherein the squares adjoin one another without gaps in a plane and wherein squares in the center of which a point light source 12 is arranged that emits light with a central wavelength are only adjacent to squares in the center of which a point light source 12 emitting light of a different wavelength is arranged.

[0098] It is also preferred that the image sensor K has, in addition to color filters, additional polarization filters with different orientations pixel by pixel on its individual pixels.

[0099] It is further preferred that the plurality of point light sources 12 for each central wavelength is an arrangement of a coherent laser source L, a Kepler telescope and a point light source array PLQA, which consists of a transparent substrate, on the light entrance side facing the telescope there are microlenses whose focus lies in the plane of a light exit side of the substrate, where there is an aperture array AA, from which object waves with a central wavelength emanate.

[0100] A further preferred embodiment is characterized in that the interferometer according to one of claims 1 to 8, characterized in that the plurality of point light sources 12 has, for each central wavelength, an arrangement of optical fiber ends which is coupled by one or more optical couplers 8 to a laser source 6 emitting light of the central wavelength.

[0101] In a preferred embodiment of the method, the optically smooth test specimen 122 is illuminated with multiple illumination configurations 14.1, 14.2, 14.3, wherein each illumination configuration results from a plurality of object waves with which the test specimen is illuminated. Object waves influenced by the test specimen 122 by reflection at the surface 108 to be measured or by transmission through the test specimen 122 with reference waves coherent with the object waves are superimposed on at least one image sensor K to form interferograms. Each illumination configuration is realized by its own color channel or its own central wavelength, wherein a first of the illumination configurations has only object waves 14.1, 14.2, 14.3 that have a first central wavelength, and wherein a second of the illumination configurations has only object waves 14.1, 14.2, 14.3, which have a second central wavelength, wherein a third of the illumination configurations has only object waves 14.1, 14.2, 14.3, which have a third central wavelength, wherein the first central wavelength is different from the second central wavelength, wherein the third central wavelength is different from the first central wavelength and from the second central wavelength, and wherein the test object (122) is simultaneously illuminated with object waves of the first illumination configuration and the second illumination configuration and the third illumination configuration from discretely different directions and, after an interaction with the test object 122, object waves emanating from the latter are superimposed on the image recorder K, and wherein the interferograms resulting from the superposition are spectrally broken down into wavelength-individual partial interferograms.

Claims

1. Interferometer (10) for the measurement of a surface (108) or an optical thickness of an optically smooth test object (122), the interferometer (10) being configured to illuminate the optically smooth test object with a plurality of illumination configurations, each illumination configuration being produced by a plurality of object waves (14.1, 14.2, 14.3) with which the test object is illuminated, and to superimpose object waves influenced by the test object by means of reflection on the surface (108) to be measured or by means of transmission through the test object (122) onto reference waves coherent with the object waves (14.1, 14.2, 14.3) on at least one image capture device (K), to produce interferograms, characterized in that each illumination configuration is realized by object waves having wavelengths of a continuous wavelength region which has a central wavelength that is specific to the wavelength region, a first of the illumination configurations having only object waves (14.1, 14.2, 14.3) of a wavelength region having a first central wavelength, and wherein a second of the illumination configurations (14.1, 14.2, 14.3) has only object waves of a wavelength region which has a second central wavelength, and a third of the illumination configurations having only object waves (14.1, 14.2, 14.3) of a wavelength region which has a third central wavelength, the first central wavelength being different from the second central wavelength, the third central wavelength being different from the first central wavelength and from the second central wavelength, object waves which propagate in closely adjacent directions not containing wavelengths of the same wavelength region, and the interferometer (10) being configured to illuminate the test object (122) simultaneously with object waves of the first illumination configuration and the second illumination configuration and the third illumination configuration from discretely different directions, and, following an interaction with the test object (122), to superimpose object waves returning therefrom on the image capture device (K), and the image capture device (K) being configured to spectrally decompose the interferograms produced by the superposition into wavelength-specific partial interferograms.

2. Interferometer (10) according to claim 1, characterized in that a fourth of the illumination configurations has only object waves of a wavelength region which has a fourth central wavelength, the fourth central wavelength being different from the first central wavelength, the second central wavelength and the third central wavelength, and the interferometer being configured to illuminate the test object simultaneously with object waves of the first illumination configuration and the second illumination configuration and the third illumination configuration and the fourth illumination configuration from discretely different directions and, following an interaction with the test object, to superimpose object waves returning therefrom on the image capture device, and the image capture device being configured to spectrally decompose the interferograms produced by the superposition into wavelength-specific partial interferograms.

3. Interferometer according to any of the preceding claims, characterized in that the interferometer has a plurality of point light sources from which the object waves of an illumination configuration are emitted simultaneously.

4. Interferometer (10) according to any of the preceding claims, characterized in that the image capture device (K) is configured to separate interferograms using color filters, such that each pixel of the image capture device (K) detects only the interferogram information of only one wavelength region.

5. Interferometer (10) according to claim 3 and at least one other of the preceding claims, characterized in that the point light sources (12) are arranged such that each point light source (12) is arranged in the center of an imaginary regular hexagon, the hexagons being adjacent to each other without gaps in one plane, and such that hexagons, in the center of which a point light source (12) is arranged which emits light with wavelengths of one of the wavelength regions with a central wavelength, is only adjacent to hexagons in the center of which a point light source (12) which emits light of a wavelength of another of the wavelength regions.

6. Interferometer (10) according to claim 3, characterized in that the point light sources (12) are arranged such that each point light source (12) is arranged in the center of an imaginary regular square, the squares being adjacent to one another without gaps in one plane, and squares, in the center of which a point light source (12) is arranged which emits light with wavelengths of one of the wavelength regions which has a central wavelength, is only adjacent to squares in the center of which a point light source (12) is arranged which emits light of a wavelength of another of the wavelength regions.

7. Interferometer (10) according to claim 4, characterized in that the image capture device (K), in addition to color filters, additionally has polarization filters on its individual pixels, with pixel-wise different orientations.

8. Interferometer (10) according to claim 3 and at least one other of the preceding claims, characterized in that the plurality of point light sources (12) for each of the wavelength regions is an arrangement of a coherent laser source (L), a Keplerian telescope, and a point light source array (PLQA) which consists of a transparent substrate, there being micro-lenses on the light entry side thereof facing the telescope, and the focal point thereof is in the plane of a light exit side of the substrate, where an aperture array (AA) is located, from which the object waves of one of the wavelength regions are emitted.

9. Interferometer according to claim 3 and at least one other of claims 1 to 8, characterized in that the plurality of point light sources (12) for each of the wavelength regions has an arrangement of optical fiber ends coupled by means of one or more optical couplers (8) to a laser source (6) emitting light of the central wavelength.

10. Method for the measurement of a surface (108) or an optical thickness of an optically smooth test object (122), the optically smooth test object (122) being illuminated with a plurality of illumination configurations (14.1, 14.2, 14.3), each illumination configuration being made up of a plurality of object waves with which the test object is illuminated and which are emitted by the plurality of point light sources (12), and object waves influenced by the test object (122) by means of reflection on the surface (108) to be measured or by means of transmission through the test object (122) being superimposed onto reference waves coherent with the object waves on at least one image capture device (K), to produce interferograms, characterized in that each illumination configuration is realized by means of object waves with wavelengths of a continuous wavelength region which has a central wavelength that is specific to the wavelength region, a first of the illumination configurations having only object waves (14.1, 14.2, 14.3) of a wavelength region which has a first central wavelength, and a second of the illumination configurations having only object waves (14.1, 14.2, 14.3) of a wavelength region which has a second central wavelength, a third of the illumination configurations having only object waves (14.1, 14.2, 14.3) of a wavelength region which has a third central wavelength, the first central wavelength being different from the second central wavelength, the third central wavelength being different from the first central wavelength and from the second central wavelength, object waves which are emitted by two closely adjacent point light sources not containing wavelengths of the same wavelength region, and the test object (122) being simultaneously illuminated with object waves of the first illumination configuration and the second illumination configuration and the third illumination configuration from discretely different directions, and, following an interaction with the test object (122), object waves returning therefrom are superimposed on the image capture device (K), and the interferograms produced by the superposition being spectrally decomposed into wavelength-specific partial interferograms.